I watch a lot of movies and tv on my ipad2 so I wanted as much storage as I could get...I'm quite happy with the 64 GB and now that I have the amazing Netflix app, I don't think I will use all 64 Gigs...
I first wanted an 16GB, but I got the 32GB model as a present and I'm glad since I have some movies and series on it and my music library is also pretty large... So it's well fed ^^
Got the 16GB a couple of week ago. Didn't put much music on (got iPhone for that) so that's only 1GB. Got 2 GB of photos and 1GB of apps. Still got plenty of room to add videos if I'm going away (got the VLC app for that).
So much depends on what you want the iPad for. If I had the 32GB I'd never fill it. Probably only half fill the 16GB at this rate!